Have you considered something more direct, such as a subwoofer amp that includes a remote gain control, or putting an inline stereo attenuator on the RCA's to the subwoofer amp? If you do that, you can (potentially) wire it so that you get your front/rear fader back. (As illustrated).

In the meantime, have you tried:

- Adjust your subwoofer's gain down a bit, and put your fader in the center as the "normal" position. That way, when you adjust the balance between fronts and sub, it's smoother because you're not suddenly "turning the sub on", but rather just decreasing the fronts a bit.

- Use the Hijack bass control and the player's loudness control to alter the amount of low frequency without altering the fade (only works if you've put the fader in the center).